Using ozone injection to control HCCI combustion

28 August 2013

In-cylinder pressure (on the left) and heat release rate (on the right) as a function of the CAD and ozone concentration injected in the intake of the engine for PRF40. The results show that ozone seeding improves combustion and advances phasing. The same trends were observed for the other fuels studied. Credit: ACS, Masurier et al. Click to enlarge.

HCCI (homogeneous charge compression ignition) has been extensively studied in recent years due to its potential to maintain strong efficiency similar to compression ignition (CI) engines but also to produce very low emissions of NOx and particulate matter. Control of HCCI autoignition and combustion phasing is challenging, however. Unlike CI and spark ignition (SI) engines, HCCI cannot be easily controlled by external means (e.g., injectors or spark plugs), because the process is mainly governed by chemical kinetics.

Accordingly, many strategies have been studied to control combustion. Now, researchers in France have added to that body of work by showing that ozone seeding in the intake of an HCCI engine can control HCCI combustion.

In a set of experiments reported in a paper in the ACS journal Energy & Fuels, they used the oxidizing potential of ozone to control the combustion phasing of the six primary reference fuels (PRFs) in HCCI. They also performed simulations in a constant volume reactor to explain the effect of ozone based on the chemical kinetics.

Their results showed that this oxidizing chemical species can improve the combustion of each fuel tested and move forward their phasing. Results on cool and main flame phasing and CA05 and CA50 showed that injection of low ozone concentrations (below 20 ppm) in the intake of the engine yield a strong combustion advance. For higher ozone additions, the combustion phasing continues to move forward but only moderately.

Unlike compression ignition (CI) and spark ignition (SI) engines, HCCI cannot be easily controlled by external means, such as injectors or spark plugs, because the combustion progress is mainly governed by chemical kinetics. This is why many strategies have been studied to control the start of combustion and its phasing.

...Because HCCI combustion is governed by chemical kinetic mechanisms, the use of oxidizing chemical species has been proposed. Among the many oxidizing chemical species, ozone is probably one of the most promising promoters of combustion. Several studies used this molecule in various applications, such as burner setup and internal combustion engines.

—Masurier et al.

The researchers, from Université d’Orléans France and the Centre National de la Recherche Scientifique (CNRS)−Institut des Sciences de l’Ingénierie et des Systèmes (INSIS), used a 0.5L single-cylinder research engine with a compression ratio of 16:1. To create the six PRFs, they combined iso-octane an n-heptane with different volume percentages in increments of 20 percentage points. For example, PRF0 contained 0% iso-octane and 100% n-heptane; PRF20 contained 20% iso-octane and 80% n-heptane, and so on up to PRF100 with 100% iso-octane and 0% n-heptane.

Ozone was produced with an ozone generator.

Their experimental setup allows the control of several parameters: engine speed; equivalence ratio; intake thermodynamic conditions (pressure and temperature); and ozone. For this study, the engine speed was fixed at 1500 rpm and the equivalence ratio at 0.3.

The...results show that ozone addition lead to earlier fuel combustion. For all of the fuels, except PRF100, where any cool flame occurs on the heat release rate, the effect is more pronounced for the main flame than for the cool flame. This can be explained by the fact that the phasing of the reference combustion used is delayed because of the limited conditions (i.e., without ozone and for limited conditions of intake pressure and temperature).

Generally, the advance on the combustion phasing is very visible for ozone concentrations lower than 20 ppm, and the effect is slighter for higher concentrations. Moreover, it can be seen that, for all of the fuels studied, cool flame phasing and main flame phasing follow the same trend, taking into account the standard deviation for each flame phasing, 0.1 and 0.25 CAD, respectively. For ozone concentrations higher than 20 ppm, phasing between each flame follows linear and parallel trends.

Unlike most of the rather useless research mentioned on these pages, this research regarding chemical control of HCCI, has potentially huge impacts. Efficiency and cleanliness improvements from practical HCCI operation, would propel improvements for all automotive and even some non-automotive applications.

Translation: we might gain have significant improvements and savings in both fuel economy and air quality, and also cost of automobiles. Imagine a car that has minimal emission equipment, yet still is clean enough to be a ZEV, while obtaining 50 mpg.